The invention relates to the field of joining copper foils and separator sheets for pressing multilayers.
It is known that the outer layers of a multilayer coming off the press consist of a full-size copper foil which is laminated to the underlying prepreg (a glass fabric reinforced with epoxy resin). The final image pattern is then etched off these foils at a later stage. To be able to do this, it is important (for the end-user) to have a copper foil that is totally fee from defects like dents, scratches and bumps. Such an even surface can only be achieved by using separator sheets with a completely smooth surface. This also enables several books to be pressed at one time.
Lamination of multilayers takes place in multi-daylight or vacuum presses, whereby the copper foil is laminated to the prepreg at about 180xc2x0 C. under pressure, thus forming an inseparable compound with the inner layers.
According to DE 38 44 498 A1, distortion of the laminated multilayers causing an uneven copper surface can be avoided if the lamination is carried out using a vacuum in combination with an isostatic pressing technique. For this, additional floating plates are inserted which are thermally insulated to keep the press plates cool when the multilayer is heated.
A disadvantage of this technique is that each book (on each of the floating sheets) must be the same size and thickness, which means that the edges of the multilayer can be easily contaminated by the epoxy resin flowing out during heating.
According to DE 35 07 568 C2, a gliding protector is to keep the prepreg layers in position. However, this can only work if oversize copper foils are used so that the resin can run off on them. The disadvantage here is that this technique uses stainless steel. Since stainless steel is a poor conductor of heat, it takes more time and thus requires more energy to heat the books. Moreover, the heat distribution is worse and less uniform than in the case of aluminum, for example.
The technique according to DE 41 16 543 A1 uses a steel sheet separator with a thermal expansion value increased to 16xc3x9710xe2x88x926 C per degree Celsius to bring it close to the expansion value of copper. This certainly minimizes surface tension, but the disadvantage of a longer heating time still remains.
As a consequence, aluminum sheets are used more and more frequently instead of steel, since aluminum conducts the heat better and more evenly than steel.
A further disadvantage of all the techniques described above is that each book has to be assembled in a lay-up room by specially trained staff, whereby each copper foil and separator must be combined with the other layers manually one by one (see Manfred Huschka, xe2x80x9cEinfxc3xchrung in die Multilayer-Prexcex2technikxe2x80x9d, Eugen G. Leuze Verlag 1988, chapter 3). Delicate, ultra thin copper foils, in particular, are easily damaged when handled in this way.
The technique according to DE 31 31 688 A1 merely describes the production of a copper-aluminum compound, making no mention at all of the separate handling of the copper foils.
During the subsequent lamination process, there is also a danger of considerable surface tension occurring as a result of the bonding and, in particular, the different thermal expansion values of copper and aluminum, which means that the aluminum starts to expand much earlier than the copper. Moreover, liquid epoxy resin may also contaminate the edges of the books in this case.
The technique described in U.S. Pat. No. 5,160,567 also deals with the production of laminates. Here, the stainless steel plates are covered with an oversize copper foil on both sides in a first step, then the foils are joined to each other along the edges of the plate.
This technique, too, has its disadvantages, as the heat conduction and distribution is worse (and more uneven) than in the case of aluminum, which means that more heating time is required. Moreover, the copper foils also have to be applied manually one by one. Especially when using ultra-thin foils, however, this can hardly be done without kinking/damaging the foils.
The invention is therefore based on the task of developing a technique that prevents or largely avoid both the sticking together of the various books and a deformation of the copper foils as easily caused if additional operations are required.
This task is achieved in the invention by the features as follows. Copper foils of any give type and thickness with aluminum separators of any give alloy and thickness can be connected together for use in laminating multi-layers by placing the copper foil panel on both sides of the aluminum separator. The copper foils are larger than the aluminum separator so that the copper foils project beyond the edges of the aluminum separator on all sides. Further the copper foils lie flat on the aluminum separator. Fixing the oversized and projecting copper foils to each other in such a manner that there is a free space between the fixation points and aluminum separator allows the separator to expand and, as noted above, prevents deformation of the copper foils. Subsequent punching of registration slots can be provided if required.